Page 40 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 39
within the lower part of the andesite flows, or within 1500 to 2000 feet of the land surface at that time. This feature is illus- trated at Hillsboro and in the Sierra Cuchillo, but in other dis- tricts the andesite, and with it the top of the intrusive mass, has been planed off by erosion down to the Paleozoic sediments. Faulting continued throughout this period along the regional fractures and between the faulted blocks, which were further tilted and dislocated. The pushing up of the intrusive masses arched the overlying beds, and in the overlying and surrounding rocks radial fractures were developed. Arching is not sufficient, however, to account for the volume of intrusive rock that has become emplaced within the sediments, and it is considered more lil:cly that these masses to a large extent worked their way toward the surface by crosscutting and stoping through the sediments, with partial or complete assimilation of the detached blocks.
The next important event in the geological succession was the formation of dikes, small sills, and small surface flows of latite and latite porphyry. These rocks occur close to the known igneous intrusions, largely within the radial system of fractures developed by the intrusives, or, as at two places near Chloride and in the Macho district, the veins occur in radially arranged fractures in the andesite, beneath which intrusive rocks are strongly suspected. The latites have the same mineralogical composition as the intrusive monzonites, and are considered to be apophyses from these main masses and a part of the same magma, introduced from depths through the upper and outer shell of solidified monzonite. The introduction and crystallizing of these latitic rocks was followed by fracturing and movement along the dike walls, with continued movement along the regional faults and those of lesser magnitude. Ore-bearing solutions from the magma chambers were thus able to migrate upward and outward from these intrusive masses, and according to distance from the source of supply, conditions of pressure and tempera- ture, nature of the wall rock or of the intermingling solutions, or as a result of the structural features existing, they formed the copper-gold-silver deposits of the Chloride and Hillsboro dis- tricts, the lead-silver and lead-manganese-silver deposits of the Hermosa, Kingston, Lake Valley and Macho districts, or the iron, lead-zinc, copper-silver and other deposits found scattered throughout the county.
Subsequent to the formation of these mineral deposits there was a period of active erosion, during which lenses of detrital material were laid down in low areas on the surface of the ande- site flow rocks. Following this interval enormous quantities of rhyolite were poured over the surface, surrounding and com- pletely covering the faulted blocks of older sediments and ande- site flows, in some places to depths approaching 2000 feet. Ero- sion then became active, and throughout Miocene and Pliocene